In current telecommunication systems, information is sometimes exchanged in a digital format. The digitizing of such information may produce a complex digital signal, such as a quadrature amplitude modulation signal comprising an in-phase component (I) and a quadrature component (Q), that may then be applied to multiple DACs for conversion to an analog signal. The analog signals output by each of the multiple DACs are then combined after being modulated onto a radio frequency carrier and amplified for transmission over an air interface. In order to coherently combine the outputs of the multiple DACs, the outputs of the DACs should be nearly perfectly phase aligned, that is, preferably should have a phase differential of less than four nanoseconds (ns) where the DAC component of this differential should be less than 0.5 ns.
It is well known in the art that a digital-to-analog converter (DAC) comprises a digital portion and an analog portion. Propagation of a signal through the analog portion introduces a delay, or a skew, to the propagation of the signal. Further delay is introduced by the insertion of the signal into the analog portion. In order to coherently combine the outputs of the multiple DACs, the analog path of each DAC needs to be nearly perfectly aligned. However, due to DAC component integrated circuit process variations and variations among the DACs in operating temperatures, age, and supply voltage, the delay introduced to a propagating signal varies from DAC to DAC. This delay difference among DACs, also known as a DAC-to-DAC skew, makes coherent combining difficult at best. Currently, the only method for controlling DAC-to-DAC skew is to carefully control the manufacture of DACs in order to assure that each DAC is nearly identical to all other DACs and/or to test DACs for their skew in order to identify DACs with identical skew. However, not only is this a costly and burdensome process, but this also fails to resolve phase differences that may result from variations in operating temperatures, age, and supply voltage among multiple DACs that may reside on different boards but whose outputs are being combined.
Therefore, a need exists for a method and apparatus that minimizes DAC-to-DAC skew when the DACs are employed in a non-ideal operating environment.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.